Amplifier system



Jan. 16, 1945. w NELSQN 2,367,600

'AMPLIFIER SYSTEM Filed NOV. 5, 1941 w '5 r T 1 & a

INPUT INVENTOR WALTER H. NELSON Patented Jan. 16, 1945 l AMPLIFIERSYSTEM Walter 11. Nelson, Cadillac, Mich assignor to FarnsworthTelevision and Radio Corporation, a corporation of Delaware ApplicationNovember 3, 1941, Serial No. 417,589 4 Claims. (01.179-171) Thisinvention relates to an amplifier system and particularly relates to anew and improved stabilized amplifier system.

is not restricted to this number of tubes.

In-conventional amplifiers, a plurality of amplifler tubes are connectedin cascade relation,

that is, the output circuit of one tube is always coupled to the inputcircuit of the following tube. In such amplifiers, a common voltagesource is provided for supplying operating voltages to all of theamplifier tubes. This common voltage supply represents a connectionbetween the input and output circuits of the various amplifier tubesand, therefore, such amplifiers tendto oscillate at a low frequency,which effect is generally termed "motor boating." This instability andthe oscillations due thereto can be suppressed by roviding by-passcondensers of extremely large capacity and de-coupling resistors of veryhigh value.v The use of very large condensers is undesirable because ofcost and space requirements, while the use of very high de-couplingresistors is undesirable because of the losses incurred thereby.

It is the object of the present invention, therefore, to provide a newand improved amplifier been shown, it is understood that the inventli gne circuits in which tubes I and 2 are connected are of conventionaldesign for amplifier tubes including screen grids and suppressor grids.However, they will be briefiy described. The input terminals 4 and 5 arecoupled to the control grid of the tube I for applying the input signalthere to by means of a coupling condenser 8 and a grid leak resistor 9.For developing the desired 'operating bias voltage for the tube, thereis connected in its cathode circuit a resistor Ill shunted by a by-passcondenser II. The suppressor grid of tube I is directly connected to thenegative terminal of a common voltage source schematically indicated bya battery l2. The screen grid of tube 1 is held at a predeterminedpotential by connecting the same to the positive terminal of voltagesource I2 through a resistor I3. For

by-passing alternating screen grid currents,

there is provided a condenser I4 connecting the screen grid with thenegative terminal of the voltage source I2. Finally, the anode of tube Iis connected by way of a load impedance includsystem of increasedstability, high efilciency, and

low cost.

In accordance with the present invention, there is provided an amplifiersystem, comprising a plurality of amplifier tubes connected in cascaderelation, each having an anode. A common source of operating voltage isprovided for all anodes. A load impedance is connected to the anode ofone of the tubes and a means including an impedance element forconnecting the anode through the load impedance to the common voltagesource. A second one of the tubes immediately following the first-namedtube in the amplifier system is provided with an electrode adapted tocollect a portion of the electron stream in the tube and, also, adaptedto be maintained at a substantially steady potential. Finally, means,including a selective network and the above-mentioned impedance element,are provided for connecting the electrode of the second tube to thecommon voltage source.

For a better understanding of the invention, together with other andfurther objects thereof, reference is had to the following description,taken in connection with the accompanying drawing, and its scope will bepointed out in the appended claims.

ing a resistor I5 and an inductance element I8, connected in seriesrelation, and a de-coupling resistor I! to the positive terminal of thevolt-' coupling resistor and the common voltage source.

The single figure of the accompanying drawing The output circuit of tubeI is coupled to the control grid of tube 2.. Since the circuit of tube 2is the same as that of tube I, it is not deemed necessary to describethe same in detail and corresponding elements thereof are indicated inthe drawing by the same numerals, accompanied by a distinguishing index.

The output circuit of tube 2 is coupled to the control grid of tube 3and the circuit of tube 3 is the same as the circuits of tubes I and 2,with the exception-oi a. distinguishing feature. Since the circuit oftube 3 is similar tothose already described, it is not believednecessary to repeat the description, and corresponding circuit elementsare indicated in the drawing by the same numerals accompanied by adistinguishing index. While the screen grids of tubes I and 2 areconnected by way of resistors l3 and I3, respectively, to the positiveterminal of the common voltage source I2, it will be noted that thecircuit of tube 3 is not provided with such a connection. Instead, thereis provided a resistor I9, connecting the screen grid of tube 3 .withthe junction point of inductance element I 6' and resistor II in theoutput circuit of tube '2. The resistor I9 in combination with conedenser-s I4" and I8 forms a selective network passing certain lowfrequencies only and attenuating all others.

In operation, an input signal is applied to the input terminals 4 and 5,which is amplified in the conventional manner by means of tubes I, 2 and3, to develop an amplified signal across the output terminals 6 and I.In case of a shock excitation, such as may be due to the surge,occurring when initially switching on the amplifier system or which maybe due to any other type of surge, a voltage pulse could be produced inthe output circuit of tube 3 which, by way of the common voltage source12, would be applied in proper phase to the input circuits of certainother amplifier tubes and cause the amplifier to oscillate. A negativevoltage pulse across the load impedance comprising resistor andinductance element l6" of the tube 3 would be caused by a positivevoltage pulse appearing in the output circuit of tube 2, which pulse isapplied to the control grid of tube 3. Due to the presence of the novelfeature according to this invention, however, a positive pulse .in theoutput circuit of tube 2, is also applied to the screen grid of tube 3,since condenser l8 represents an appreciable impedance at lowerfrequencies, at which the oscillations occur. As the screen grid currentcharacteristic of a tube is much like the anode current characteristicof a triode tube of low plate resistance, a relatively small change inscreen grid voltage effects a relatively large change in screen gridcurrent. Therefore, the positive pulse applied to the screen grid oftube 3 causes a current pulse to fiow through the cathode screen gridspace of tube 3, resistor 19, resistor I I and voltage source 12, and anegative voltage pulse is developed across resistor H which tends tocounteract the positive pulse applied to the control grid of tube 3 bymaking the junction point of resistor l1 and resistor IS a commonpotential point at one terminal of the plate load of tube 2 consistingof resistor l5 and inductance element 5'. In this manner, any tendencyof the amplifier system to oscillate at a low frequency is eliminated.In order not to degenerate the useful signal applied to the tube 3, thefrequency selective network comprising condenser l4", resistor l9 andcondenser I8 is so designed as to pass the low frequencies only at whichthe amplifier system tends to oscillate.

While the same novel feature could be inserted in other stages of theamplifier system, that is, the connection of the screen grid of thefollowing tubes by way of selective network to the output circuit of thepreceding tu-be, this was not found necessary since the greatest dangerof oscillation generation exists in the last stage of an amplifiersystem, since the signal level there is the highest. If found necessary,however, similar connections could be made between other stages of theamplifier system.

The particular embodiment of the invention is a television picturesignal amplifier system, in which the following tubes and circuitconstants were found to operate effectively.

' the condensers and resistors used all have relatively small values.

While there has been described what is at present considered thepreferred embodiment of the invention, it will be obvious to thoseskilled in the are that various changes and modifications may be madetherein without departing from the invention, and it is, therefore,aimed in the appended claims to cover all such changes and modificationsas fall within the true spirit and scope of the invention.

What is claimed is:

1. An amplifier system comprising a plurality of amplifier tubesconnected in cascade relation, each having an anode, a common source ofoperating voltage for said anodes, a load impedance connected to theanode of one of said tubes, means including an impedance element forconmeeting said anode through said load impedance to said voltagesource, an electrode in a second of said tubes immediately followingsaid firstnamed tube in said amplifier, said electrode being adapted tocollect a portion of the electron stream in said second tube and alsoadapted to be maintained at a substantially steady potential, and meansincluding a selective network connected between said load impedance andsaid impedance element for connecting said electrode to said voltagesource.

2. An amplifier system comprising a plurality of amplifier tubesconnected in cascade relation, each having an anode, a common source ofoperating voltage for said anodes, a load impedance connected to theanode of one of said tubes, means including an impedance element forconnecting said anode through said load impedance to said voltagesource, a screen grid electrode in a second of said tubes immediatelyfollowing said first-named tube in said amplifier, and means includingsaid impedance element for connecting said electrode to said voltagesource and said load impedance and developing a pulse to counteract apulse of the opposite polarity in said load impedance.

3. An amplifier system comprising a plurality of amplifier tubesconnected in cascade relation, each having an anode, a common source ofoperating voltage for said anodes, a load impedance connected to theanode of one of said tubes, an electrode in a second of said tubesimmediately following said first-named tube in said amplifier, saidelectrode being adapted to collect a portion of the electron stream insaid second tube and also adapted to be maintained at a substantiallysteady potential, and means including an impedance element forconnecting said electrode and said load impedance to said voltage sourceto develop a feed-back voltage in said load impedance.

4. An amplifier system, comprising a plurality of amplifier tubesconnected in cascade relation, each having an anode, a common source ofoperating voltage for said anodes, a load impedance connected to theanode of one of said tubes, an electrode in a second of said tubesimmediately following said first-named tube in said amplifier, saidelectrode being adapted to collect a portion of the electron stream insaid second tube and also adapted to be maintained at a substantiallysteady potential, and means including a low pass I filter and animpedance element for connecting voltage source.

From the above given values, it may be seen that said electrode and saidload impedance to said WALTER H. NELSON.

